YouMail - YouMail turns visual voicemail into a personal assistant for handling all your missed calls.
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Hi Voicemail - Stop robocallers filling up your voicemail
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
MightyCall - Virtual phone system that lets you run your business from anywhere.
Sipdroid - Free and open-source SIP/VoIP Client